1. Field of the Invention
The present invention relates to a method of forming a photovoltaic element. In particular, the present invention relates to a method of forming a photovoltaic element which enables the solution of the problem caused by irregularities formed on a metal layer surface of a photovoltaic element, in other words, the problem of defective portions appearing in a photovoltaic element because the photoelectric converting layer formed on the irregularities of the metal layer surface cannot fully cover the metal layer due to the existence of projecting portions or portions with great height difference in a part of the irregularities. The present invention is suitably applied particularly when forming photovoltaic elements such as solar cells and sensors utilizing non-single-crystalline semiconductors such as of amorphous silicon, microcrystalline silicon, amorphous silicon-germanium alloy and polycrystalline silicon.
2. Related Background Art
In photovoltaic elements for use in solar cells etc., their metal layers and metal oxide layers, which are provided under semiconductor layers functioning as photoelectric converting layers, have been devised in various ways in order to effectively utilize the light with which the photovoltaic elements are illuminated. In Japanese Patent Application Laid-Open No. 4-218977, for example, there is disclosed a technique in which reflection at a light reflection layer is improved by depositing a discontinuous metal layer having irregularities thereon and a continuous metal oxide layer of a uniform thickness on the above metal layer. However, in order to improve the photoelectric conversion factor (efficiency) of the photovoltaic element, further improvement in reflection is desired.
In Japanese Patent Application laid-Open No. 6-116722, there is disclosed a method of forming metal layers and transparent electrode layers continuously by the sputtering method while moving a long substrate (generally referred to as Roll to Roll method). With a system for forming such layers to which this method is applicable, devices can be formed continuously without stopping the system for hours, thereby high productivity can be obtained.
In Japanese Patent Application Laid-Open No. 9-92857, there is disclosed a method of forming a satisfactory photovoltaic element in which a reflection layer is deposited on a supporting member, the supporting member with the reflection layer is cooled to 100xc2x0 C. or lower before stacking a reflection increasing layer thereon, so as to control oxidation of the reflection layer surface and improve the films closely touching with each other.
In the methods of forming a photovoltaic element according to the prior arts, when continuing deposition for hours, a problem as described below arises, due to a change in depositing conditions with time, which makes it hard to obtain a photovoltaic element with high reliability and stable performance.
As one of the methods of forming thin films such as a metal layer, the sputtering method is used in which a high energy is applied to a bulk material to form its particles and the particles are deposited on a supporting member to form a thin film. In this case, however, the deposited particles lose most of their energy in an instant; accordingly, thin films are often formed in an instance before they reach thermal equilibrium by this method.
On the other hand, a metal layer provided under a semiconductor layer functioning as a photoelectric converting layer can effectively reflect the light having transmitted the photoelectric converting layer if its surface has irregularities. As described above, however, since the metal layer is formed before its particles reach thermal equilibrium, there are likely to exist remarkably projecting portions or portions with great height difference between the concavities and convexities in a part of the irregularities; as a result, defective portions may be generated because, for example, the photoelectric converting layer formed on the irregularities cannot fully cover the metal layer. In such a photovoltaic element formed on a supporting member, there has been a problem of short-circuit current being generated at the defective portions.
Accordingly, the object of the present invention is to provide a method of forming a photovoltaic element which enables the solution of the problem caused by the irregularities formed on a metal layer surface of a photovoltaic element, in other words, the problem of defective portions appearing in the photovoltaic element because a semiconductor layer formed on the irregularities of the metal layer surface and functioning as a photoelectric converting layer cannot fully cover the metal layer due to the existence of remarkably projecting portions or portions with great height difference in a part of the irregularities. As a result, the present invention provides a method for stably forming a photovoltaic element of high quality, good uniformity, and high reproductivity which produces less defects and causes no short circuit in the current flow generated on the semiconductor layer.
The method of forming a photovoltaic element according to the present invention comprises at least the steps of: depositing a metal layer on a supporting member; depositing a metal oxide layer on the above metal layer; and arranging at least one or more pin structures, which is formed by stacking n-type, i-type and p-type silicon-containing non-single-crystalline semiconductor layers, on a substrate formed by stacking the metal layer and the metal oxide layer on the above supporting member, wherein a step of subjecting the supporting member having the above metal layer formed thereon to heat treatment is carried out between the step of depositing the above metal layer and the step of depositing the above metal oxide layer.
Providing the step of subjecting the supporting member having the above metal layer formed thereon to heat treatment restricts the generation of the projections and the great height difference in a part of the irregularities formed on the surface of the metal layer; therefore, a semiconductor layer formed on the irregularities and functioning as a photoelectric converting layer can fully cover the metal layer. As a result, defective portions appearing on the semiconductor layer are decreased, which enables the stable formation of a photovoltaic element which does not cause short circuit in the current flow generated on the semiconductor layer.
The above heat treatment step is preferably a step in which cooling treatment, heating treatment and cooling treatment are carried out once or repeated plural times in this order.
In the above, the temperature decreasing speed of the supporting member in the above cooling treatment is preferably 1xc2x0 C./sec or higher and 50xc2x0 C./sec or lower, the temperature increasing speed of the supporting member in the above heating treatment is desirably 10xc2x0 C./sec or higher and 100xc2x0 C./sec or lower, and the temperature for heating the supporting member in the above heating treatment is suitably 100xc2x0 C. or higher and 400xc2x0 C. or lower.